pH is a measure of the acidity or alkalinity of a solution and can be determined using a pH-selective electrode (for instance, pH glass electrode, hydrogen electrode, quinhydrone electrode, ion sensitive field effect transistor). pH sensors measure ion content and are often utilized in industrial process control systems to measure the hydrogen (H+) or hydroxyl (OH−) ion content of a solution.
pH sensors commonly employ at least two electrodes, an ion-specific electrode (a pH electrode) and a reference electrode. In one example, a pH electrode utilizes a pH sensitive glass, in contact with a solution, which develops a potential (voltage) proportional to the pH of the solution. A reference electrode provides a known reference potential for the pH electrode. The difference in the potentials of the pH electrode and the reference electrodes provides a millivolt signal proportional to pH.
Over time, pH electrodes can experience aging which can result in changes to the electrical characteristics of the electrode. Electrode aging may be caused and/or accelerated by, for example, use in high temperatures, operation of the pH electrode in process media or solutions that have either high acidity or alkalinity, or incorrect handling of the pH electrode when not in use, e.g. incorrect cleaning and storage procedures. Electrode aging can cause an increase in impedance and response time, a declining slope, especially in the alkaline region, and/or a shift of the asymmetry potential, for example. Further, electrode aging can be indicative of changes in the chemical composition of the membrane glass, steady growth of the internal membrane gel layer, and/or chemically and mechanically induced degradation of the outer gel layer of the membrane during measurement and cleaning. As an electrode deteriorates, the ability of the sensor to accurately measure pH also deteriorates resulting in inaccurate and/or inconsistent pH level measurements.